In electrical power converters, the measurement of current is commonly achieved by the use of a low-value shunt resistor to generate a small voltage proportional to the current flowing through the shunt resistor. This voltage is then generally amplified for further use or measurement.
Most power converters contain an inductor or transformer as a magnetic energy storage or transmission element. The current flowing through this magnetic element is generally composed of AC and DC components. The DC component of the current, acting on the inherent resistance of the winding of the magnetic element, creates a small DC voltage drop, which is generally superimposed on AC voltages across the winding which are a consequence of AC currents acting on the effective resistance and the inductance of the magnetic element. Some of these components may be filtered away, leaving a signal that may be proportional to the current through the inductor, the average DC current through the inductor, or some related signal, depending on the transfer function of the filter.
In many applications, this filtered voltage can be used in place of the voltage developed across a shunt resistor, and the shunt resistor may thus be eliminated. Elimination of the shunt resistor can result in higher power conversion efficiency, reduced parts count, reduced system cost, or other advantages.